title: The maternal muscle determinant in the ascidian egg.
abstract: Muscle formation in ascidian embryos has been investigated for more than a century as a representative example of cell fate specification by localized maternal factors within the egg cytoplasm. Observations of colored cytoplasm in combination with micromanipulation techniques have suggested the presence of a muscle-forming factor. The molecular basis has been elucidated with the discovery of macho-1. macho-1 mRNA is already present in the unfertilized egg, and translocates to the posterior region of the egg during ooplasmic movements. It encodes a zinc-finger transcription factor that positively regulates the expression of target genes. macho-1-binding cis-elements have been identified in muscle-specific zygotic genes. Maternally localized macho-1 appears to have originated in the ascidian lineage, but it activates a muscle-forming developmental program that is shared by the vertebrates. macho-1 is also involved in establishment of the anterior-posterior axis as a competence factor in mesenchyme induction in the posterior region. It is suggested that translation of the macho-1 protein is initiated at the eight-cell stage, and that the protein is inherited by all descendant blastomeres of the posterior-vegetal region. The macho-1 activities in nonmuscle descendants are suppressed or modified by cell interactions during the cleavage stages. In addition to the primary muscle specified by maternal macho-1, ascidian embryos develop secondary muscle, whose fate is determined by cell interactions. Dozens of maternal mRNAs show similar localization to macho-1, and these are known as postplasmic/PEM RNAs, being also involved in various posterior-specific developmental events. Evolutionary aspects relevant to macho-1 and tail muscle formation are also discussed in this article.
authors.affiliation: Department of Biological Sciences, Graduate School of Science, Osaka University, Toyonaka, Osaka, Japan. hnishida@bio.sci.osaka-u.ac.jp
authors.forename: Hiroki
authors.initials: H
authors.lastname: Nishida
mesh_terms: D000818:Animals; D019143:Evolution, Molecular; D024510:Muscle Development; D009132:Muscles / Q000196:embryology* / Q000378:metabolism*; D010063:Ovum / Q000378:metabolism*; D023121:RNA, Messenger, Stored / Q000235:genetics / Q000378:metabolism; D014157:Transcription Factors / Q000378:metabolism; D014561:Urochordata / Q000196:embryology* / Q000235:genetics / Q000378:metabolism*
chemical_list: D023121:RNA, Messenger, Stored; D014157:Transcription Factors